Why does the collectionchanged event not fire in the following code, yet I can see the new instance of InventoryBTO I add to the ObservableCollection?

 private ObservableCollection<InventoryBTO> _inventoryRecords;
    public ObservableCollection<InventoryBTO> InventoryRecords
    {
        get { return _inventoryRecords; }
        set { _inventoryRecords = value; }
    }

    private InventoryBTO _selectedRecord;
    public InventoryBTO SelectedRecord
    {
        get { return _selectedRecord; }
        set 
        {
            if (_selectedRecord != value)
            {
                _selectedRecord = value;
                OnPropertyChanged(new PropertyChangedEventArgs("SelectedRecord"));
            }
        }
    }

    public InventoryViewModel()
    {
        if (_inventoryRecords == null)
        {
            InventoryRecords = new ObservableCollection<InventoryBTO>();
            this.InventoryRecords.CollectionChanged += new NotifyCollectionChangedEventHandler(InventoryRecords_CollectionChanged);
        }

        _inventoryRecords = InventoryListBTO.GetAllInventoryRecords();
    }

    void InventoryRecords_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {

    }